Question: How many numbers are in the list $ 147, 144, 141, \ldots, 42, 39?$
Answer: We want to transform the numbers to a sequence from 1 to $N$, which is easy to count.  First, we reverse the list to become $39,42,\ldots,144,147$.  Then we divide each number by 3 since the numbers are spaced 3 apart to get $13,14,\ldots,48,49.$  Finally, we subtract 12 from all of the numbers to get $1, 2, \ldots, 37.$  So there are $\boxed{37}$ numbers.